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Pronghorn Research Solves Decades-Old Evolutionary Controversy

By Tara Roberts

Ten years of in-depth research on the pronghorn antelope helped University of Idaho biologist John Byers solve an historic controversy in evolutionary biology: Does a male’s number of matings increase the number of offspring he has?

The answer, reported in Byers’ recently published paper in the journal “Science,” is yes – albeit with unexpected variables.

In “Bateman in Nature: Predation on Offspring Reduces the Potential for Sexual Selection,” Byers –Interim Chair of the Department of Biological Sciences – and former doctoral candidate Stacey Dunn use data gathered about pronghorn on the National Bison Range to determine whether a principle known as Bateman’s Slope holds true for animals in the wild.

Proffered by English geneticist Angus Bateman in 1948, the principle hypothesizes that the number of offspring a male has is proportional to how often he mates. This reinforces Darwin’s concept of sexual selection, which states that males develop competitive advantages to help them increase how often they mate – for example, pronghorns’ horns help them compete with other males, encouraging the females to select them for mating.

However, scientists who tried to replicate Bateman’s results over the years found less certain results. Some researchers observing animals in the wild found the slope held true, but their work was done only over one mating season.

Byers realized that the unusual depth and breadth of his pronghorn data – which was recorded for other research purposes – would allow him to address the controversy over Bateman’s work.

And while Byers and Dunn found that Bateman’s results stand, they also found complicating factors they didn’t expect.

“We showed that Bateman’s Slope swings all over the place,” Byers says. “It goes up and down from year to year.”

Byers and Dunn worked to determine what influenced males’ mating successes and found it was not population size, sex ratio or age structure of the herd. The only important variable was the number of pronghorn fawns killed by coyotes.

In a typical year, the herd had a few males who did the most mating, and Bateman’s Slope came out as predicted. But in years when fawn predation was high, the males’ success rates did not follow Bateman’s model – a male that mated often was not guaranteed a high number of offspring.